Permanent magnet



Aug. 30,1932; p. L. HORSBURGH ET'AL 1,375,004

PERMANENT MAGNET I Filed Jim 51. 1930 2 Sheets-She et 1 Patented M13530,1 932 V p I Q 4 UNITED STATES PATENT OFFICE enonen nonnnn an nonsnunonmummy swrr'r rnvrcx, or snnrrmnn, mmnnm, assrenons or one-mm 'ro swmrrnvrcx am) sons rmrrnnor sunr- IIELD, ENGLAND, A; BRITISH COIPANY, ANDONE-H.ALF 1'0 DABWINSV LIMITED, 01 SHEFFIELD, ENGLAND, A. BRITISH GUI! VPERMANENT MAGNET Application filed January 81,1!30, No. 424,957, and inGreat Britain August 22, 192 9.

This invention relates to permanent magclaims as soft iron for brevity)the flux nets and more particularly to permanent density in the gap canbe greatly mcreased, magnet units intended 'for use with sound and apermanent magnet unit obtained of a reproducing or transmittingapparatus of the strength comparablev with that of electrokind in whicha diaphragm 1s connected to magnet units as now commonly employed, a acoil situated in a magnetic field or gap. especially if the poles areshaped so that their Various attempts have been made to procrosssectional area decreases in the direction duce a satisfactory permanentmagnet unit of'the gap and it is a feature of our inventlon of the abovekind, but these have not been tothus make the said poles. p

m altogether successful. In cases where the In the case where the-blockis cast in one magnet unit has been built up of separate with anumberlof loop-like limbs (two, four parts it possesses certain defects,particularly or six, for example) integral with the crown, in that ithas not realized adequate magnetic they may radiate from the base andcrown strength owing to the presence of mechaniof a common or centralpole-piece torming u cal joints, or to the necessityfor assembling partof what has been termed above the body a the parts after the magnetshave been magportion, which is preferably circular incross netized,or tothe units being so costly as to section, the adjacent ends of theloop-like be commercially prohibitive exce t in spelimbs being separatedfromthe crown by the cial cases; moreover, there'is a liability ofaforesaid trepanning or similaroperation so m certain of the parts tomove and thus disturb that they are curved in conformity there thecontinuity of the gap. r with to form a numbenof segmental (zaps be-According to this invention most of these. tween the crown and theadjacent en ofthe defects are overcome by making the permalimbs. It ispreferable howevelg-that the nent magnet or ma etunit of magnet steellock be so castand the'trepanning or simir 95 cast in the form 0? p moreloop like limbs extending outwardly cent ends oi the limbs remain unitedtogether a.

a or radiall from the base and crown ofthe by a web ofimetal forming anintegral part'of body portlon of which they form an integral the castingso that the central pole-piece is part and. forming a gap around thecrown surroundedby metal constituting the outer V 30 by a trepanningzorsimilar operation to acwall of a continuous circular ga with the/' 5commodate the coil or moving part ofasound result that the ma etic fluxis su tantially reproducing or-tra'nsmitting ap aratus, the equalizedaround t e wholeof the gap; The block thus formed with the sea gap beingweb of metal moreover prevents the loo subjected toa hardening operationand sublike limbs from becoming'distorted e'speci gs sequentlymagnetized. In a magnet' unit ly duringhardening or operationssubsequent 33 thus made, and having the gap ormed as to thecasting,which might occur, andtodif- 7 above"'s'tated there are nomechanical joints ferent extents, if no web of metal were pres and thusthe .full-magneto motive force of ent. Instead of forming the circulargap by the magnet steel is available right up to the "trepanning asaforesaid it may be formed by h ,40 ap, unhindered by-any reluctancesintro- -.'casting the unit with a space between the in uced bymechanical 'joints. In this case crownflandthe adjacent "ends of thelimbs however as. the unit is. composed of magnet for softiron polepieces suitably. 'shapedito teel alone, the magnetic'flux'densityobtain-;- -form the gap. s able at the gap is b the permeability. Inthewhere the unit has only a le 6 of the magnet steehfbuti the unit ismade; oopkeimb, 88 i may hate, y so that the poles at the opposite sidesof the where lightness is adesideiatum, e

gap are. rovided withmaterial of higher tron -of t e metal, around thecrownni a perme,ab 'the magnet steel (for ex which the extendsoutwardly- (whether ample soft iron or cobalt iron and herein: castofmagnetsteel or of-magnet steel and a block having one or lar operationbe so performed that the-adjaf to after referred to in the specificationand Soft iron) maybe separatedthemfiomiby "it!!! 40 Figure Figure 7, butwith the soft iron pole a in the aforesaid trepanningor similaroperationto form a circular p surrounding the crown, the outer wall said gapbeing continuous; or the circular gap may be formed bycasting theunitwith the crown and'the ada'centends of the limbs separate and pro-'vi them with soft iron pole pieces suit-' abiy shaped to form the gap. tis desirable that in all cases the ag e- I! ate cross-sectional area ofthe indiv1 ual bs or the ores-sectional area of the single unit shouldbe equal to the cross-sectional area of the common pole piece.

The magnet steel used is preferably a high percentage cobalt steel (9%to 35% cobalt). We do not however confine ourselves to the use of thissteel as other magnet steels suitable for the manufacture ofpermanentmagnets may be employed.

In orderthat the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawings in which 7 Figures 1 and 2 arerespectively a Ian and a vertical section on theline 22 of e ure 1showin one form of the metal bloc cast wholly 0 ma et steel and prior tobeing formed with e gap," and i 3 and 4 aresimilar views showing e blockafter it has been formed with the gap.

Figures 5 and 6 are vertical sections showing a metal block cast in twometals viz. l8 magnet steel and soft iron, before and aftertrelpanningpf thega respectively.

lgure 1s a plan owing the metal block asdcast for use with soft ironpole pieces an 8 is a section on the line8-8 of shown separate from theblock and section.

F' 9 and 10 are similar views showa ing e block after it has been fittedwith the le pieces. a

Figures 11 and 12 are respectively a plan and a'vertical section on thelinens-12 of Figure 11 showing the ga ped cast metal a. block havin gasingle loope limb.

Figures 13 and 14 are respectively a and a side elevation of a toolsuitable for use in formingthe gap. in the castmetal Y blocks by atrepanning operation, and

I8 Figure'15 is an enlar ged plan view of a portion of the tool shown inFigure 13.

In F' p 1 to 4, A, Aare the loop-like limbs o the cast metal block whichextend radially from the base A and crown A of the .0 body portion orpole-piece A with which they form an integralor mechanically jointlessartfiip'rior to the formation of the gap B ,1 ande2) which issubsequently efi by a trepanning or e8 ,operationjas aforesaid, the saidsoft iron through a suitable eedin'g aper is filled. In the plane ofcontact of the two parts immediately surrounding the gap are and 6 iscast in two metals, one magnet steel,

and the other soft iron orother material of higher permeability than thema et steel. In castlng such a metal block t e a ropriate quantity-ofone metal is poure mto the mould and is followed by the second metal sothat the two metals'unite or co1nv bine at their junction during thecasting? operation as indicated by the cross hatching in 1 Figures 5.and 6. The metal block may be cast in an inverted condition in amoulding box by first pourin the molten ture into a central opening inthe mould until it reaches the requisite hei ht which is about one thirdof the height 0 the main portion" of the block. After allowing a timeinterval to elapse suflicient for the soft iron already cast to becomein a more or less semi-plastic condition the molten magnet steel 1souredinto the mould through the said fee ing aperture until the entiremould metals indicated by the cross hatching in Figures 5 and 6 mergintakes place between them. After the meta s have solidified the mouldingbox is opened and the. casting removed. The loop-like limbs A, A, thebase A and the central pole piece A, which constitute the bulk or mainrtion of the block, are of magnet steel whi e the crown A and theadjacent portions of the'limbs and web areof soft iron. The gap B istrepanned in the block in the same manner as in the case of the blockillustrated. in Fi ures 1 to 4 after the casting operation so t at theof soft iron as' shown in Fi relO. 'It is important that the said gapshould be in all cases accurately formed and as its width x will notgenerally exceed two millimeters, it is made after the operation andbefore hardenin by a trepanning or similar operation in w ich a toollike that illustrated by Figures 13to 15 is employed with advantage.This tool comprises a number of cutters 0 so carried or mounted'on a suprt or head E as to lie on or near thecircum erence of a circle thediameter of which corresponds with that of the circular p B to be made.The cute ters e are .sligh y less in width than that of the gap to bemade and are curved to con-. form to the curvature of the gap and thecutting edges eof' consecutive cuttersare on y opposite sides (seeFigure 13 so that while one cutteracts on the inner of thegap underformation the next cutter acts on the outer face thereof and so on. Thecutter sup port or head E is of cylindrical form carrying at one end thecutters and having at the other end a tapered portion E for securing itinosition in a lathe or other machine tool; he depth of the cutters issuch that there is at all times a clearance between the top of crosssection in the lower part of Figure 8,

. i. e. with the crown and the adjacent ends of the limbs separated witha central circular opening a and a central pole piece or body portion Aof reduced length. The inner edge of the opening a and the annular faceof the hollow central pole piece or body portion A are machined and theopening in the pole piece or body portion reamered out. The

' central pole piece A is provided with a soft iron extension piece orbody portion a) and a soft iron ring a is fitted to the inside of theopening a. A concentric stub a on the extension piecea enters the holein the central pole piece or body portion, the stub 11 being turned sothat it is a slack fit in the aforesaid hole and its end beingcounter-sunk. The ring a also isturned so that it is a press fit in theopening a. In order to complete the magnet unit the ring is pressed inand the extension piece is loosely placed in position inthe opening anda spacing tube of dimensions corresponding to those of the gap is placedbetween them so 'as' to maintain them concentric. The unit is thenturned upside down and placed on a rigid surface. and a toolinsertedthrough the hole in the'central pole piece or body portion andgiven a blow in order to expand the stub on the extension piece againstthe wall of the hole tomaintain it in position. The spacing tube is thenwithdrawn leaving the ring and extension piece secured in position. Itis to be observed that the rin a lies in the same lane as'the ad acenton s of the loop-like lim s'A and the associated webs a and thattherefore themagnetic lines pass from the block of magnetic steel to thesoft iron without appreciaof the ring and o the extension piece is suchvthat the cross-sectional area through which the magnetic lines passdiminishes in a directiontowards the gap, and, as the material is one ofa permeability which will allow of it, the flux density increasesaccordingly. This effect would not be obtained to the same extent bytapering the pole pieces of a magnet composed wholly of magnet steel onaccount of its relativel low permeability as aforesaid. In order to otain the best results magnetically it is desirable that the pro ortionof magnet steel to soft iron should as high as practicable and for themagnet les'to be as near as possible to the gap: I l; weight steelcomprising a of soft iron in the arrangement illustrated in Figures 7 vto 10 is about 10% of that of the magnet steel. Similar considerationsto the arrangement of Figure 6.

apply It is advisable not to reduce the proportional}! of soft iron tobelow the aforesaid figure as it might then be necessary to reduce thecross sectional'area so rapidly as to be likely to lead to a leakage ofmagnetic lines. The soft iron pole piece or pieces should be soassociated with the end or ends of the loop-like limbs or with thecentral pole piece or body portion that they form virtually an extensionthereof, that is to say,'the form-in eifect a contlnuation and are not,.or example, secured thereto L80.

in an overlapping manner. In the case of the arrangement illustrated inFigures 7 to 10 for example thesoft iron ring a is for this purposeinserted in the circular opening a 1n such a manner that-it does notunduly pro trude either above or below the adjacent ends of theloop-like limbs.

The magnetizing, which is carried out after the gap has been formed andthe block hardened, may be effected in any suitable manner.

In the example shown by Figures 11 and 12 the gapped cast metal blockconsists of a single loop-like limb A one end of which extends from thebase A of the cylindrical body portion or pole-piece h and the other endof which is of ring-like formation sur rounding the crown of thepolepiece or body portion A and constituting a continuous outer wall a ofthe circular gap B. A unit of this form may be made from a metal blockcast of two metals, or from onekhaving separate pole pieces of soft ironassociated therewith, as in the case of a metal block of the form havinga number of loop-like limbs.

It is to be understood thatthe term mag' not block referred to in theappended claims is intended to include not only a permanent magnet butalso a block for use in the manufacture of a permanent magnet. r

at we claim and desire to secure by Let ters Patent of the United Statesis roel. A magnet block of permanent magnet steel and having a bodyportion and inc ing a loop-like limb extending. outwardly or radiallyfrom the base of the body portion of which it forms an integral ormechanically jointless part and inwardl towards the crown gap around theof the body portion, an a crown of said body portion;

. 2. A magnet fb ock. of permanentmagnet 12.- steel and having a body,portiop a'j-p'lurality' 1 of loop-likeglimbs extending bdtwardly or 1*:radially from the'base of the bodyportionf Of which they form anintegral or inechani- '5 cally jointless part and inwardly towards thethe crown of said body portion.

crown of the body portion and a gap around '3. A magnet block ofpermanent mhgnet,

dy portionandlinclud mg;- a loop-like limb extending outwardly or tab -radiallyfrom the base and crown of the body portion .of which it formsan integral or mechanical'ly jointless art and a continuous gap itaround the crown o the body portion.

4. A magnet block of permanent magnet steel comprising a body portion, aplurality of loop-like limbs extending outwardly or radially from thebase and crown of the body a loop block in that part consisting-of soplurality of loop-like limbs extending. outwardly or radially from thebase of the body portion of which they form an integral part 1y orradially from the base and upwardly.

'. and inwardly towards the crown of the'body portion where they, arejoined by integral webs.

7 A block for magnet steel comprising a body portion hav- .inga lowerbase part and an upper crown part and a loop-like limbextendin'goutwardtowards' the crown of the body portion of which it forms anintegral or mechanically jointless part," the metal of the end of i the.limb adjacent to the crown forming an extension surrounding the latter.

'8. A magnet block-of permanent magnet steel comprising a body portionhaving a 40: andincluding aloo -like limb extending outwardly orradially tion of which they form an'integra 'of'thelimbs.

lower base part and an upper crown part cm the. base of the body portionof which it forms an integral part and-inwardl towards the crown and se1 rated pole pieces .of soft iron interposed tween the crown and. theend-of the limb.

*9. A magnet block of permanent steel comprising abody portion having alower base apart and anupper crown part and a plurality of loop-likelimbs extending outward- 1y or radially p and inwardlytowards the crownof the body portion and separated pole pieces of soft iron 1nterposedbetween the crown and the ends 10. The manufacture of a permanent magnet; or magnet unit which consists in casting a block of magnet steelintegral with a number of loop-like limbs extending outwardly orradially from the base of a body portion and inwardly towards the crownthereof,

for'min said block with a" gap around the crown t 'ere'ofoli a machiningoperation, subthus formed with said gap jectin'g the b a magnet ofpermanent froin'the base of the'body .por-

to a hardening operation, inserting soft iron po e pieces'in said gapandsubsequently magnetizing the bloc 11. A magnet ing mainly of permanentmagnet steel and portion having a lower base part and an upper crownpart and including a loop-like limb extending outwardly or radially fromthe base of the'body portion ofwhich it forms an integral ormechanicallyjointless part and inwardly towards the crown ofthe web and a gap insaid'blockin that part consisting of soft iron.

12. A magnet block of permanent magnet steel, comprising a vbodysportionhaving a lower base part and an upper crown part and including aloop-like limb extending outwardly or radially from the base of the bodyportion of whichit forms an integral part and inwardly towards the crownand a pole piece of soft iron interposed between the crown and the endof the limb.

13. A'magnet block of permanent magnet block of'cast metal consistbodyportion where it is joined by an integral steel comprising a bodyportionhaving a low- 1 er base part and an upper crown part andincluding a loop-like limb'extending outwardly or radially from the baseof the body portion of hich it forms an integral part and inwardlytowards the crown and separated pole pieces consisting of a ring and ofan extension piece interposed between the crown and the end of the limb.

14. A magnet block of permanent magnet steel comprising a body portionhaving a lower base part and an up r crown wardly or radia y from thebase of the body portion of which it forms an inte al part and inwardlytowards the crown an a single shaped member of soft iron inter osedbetween the end'of the limb and t e crown and having an air ga therein.

- 15. A magnet bloc of permanent magnet steel comprisin a body portionand'including aloopl olike 'mb extending outwardly or radially of whichit forms 'an integral part and inwardly towards. the crown and/a polepiece ofsoft iron interposed between the crownand the end of the limbthe proportion'by weight art and including a 100 -like hmb extendmg'outm thebase of the body portion of magnet steel to that of iron beingabout ten to one.

7 at block of permanent mag net,- steel comprising a body portion andinclud in a loop-like limb extendingoutwardly orra ially from the'baseof the body portion.

of which it forms an integral 'or mechanically j ointless, art andinwardly towards the crownof, the y I pole piece interpose between theendof the limb and the CI0Wl1.tl1B soft iron portion being arrangedrelatively-to the magnet steel so that the magnetic lines pass from themagrtion and a soft iron 10' partly .of soft iron' and comprising a bodyY net steel to the soft iron without any abrupt change of direction.

17 A magnet block of permanent ma et I steel comprising a body-portionand inc udfi' ing' a loop-like limb' extending outwardly or radiallyfrom the base of the body por tion of whlch it forms an integral ormechanically jointless art and inwardly towards the crown of the y rtionand a soft iron 10 le piece interposef between the end of the b and thecrown so that it forms extensions thereof. a '18. The manufacture of apermanent magnet or magnet unit which consists in casting a blockofmagnet steel with a loop-like limb extendin outwardly or radially fromthe base of t e body portion of which it forms an integral ormechanicall jointless part, and castin the crown of t e bod portion 7 jand the on of the 160 -like limb 0 soft iron so thatthe-two met unite attheir -]'unc-' tion during the castingoo eration.

' GEORGE ALD LEE HORSBURGIL I BERRY SWIFT/LEVEL.

